EP0070664A1

EP0070664A1 - Phase controlled regulated power supply circuit

Info

Publication number: EP0070664A1
Application number: EP82303638A
Authority: EP
Inventors: Randolph Alan Bullock; Lawrence John Mason
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1981-07-20
Filing date: 1982-07-12
Publication date: 1983-01-26
Also published as: CA1195374A; US4423478A; DE3273706D1; EP0070664B1; JPS5818719A

Abstract

A circuit is provided for regulating the application of power to a load (30) which is relatively insensitive to rapid power fluctuation of the type exemplified by typical AC line sources. In one embodiment, an AC line source (10) is applied through a rectifier (20) at a controlled rate to a tungsten lamp (30) whose operation is controlled by a transistor switching network (40). The transistor operation, in turn, is controlled by comparing (60) a portion of the rectified signal with a signal (from 50) proportional to the light output of the lamp (30).

Description

The present invention relates generally to the regulation of power to certain loads which are insensitive to rapid fluctuation of power and, more particularly to a phase controlled regulated power supply cricuit which controls the conduction period so that power to a load, is applied and removed, at a controlled (slowed-down) rate.
Systems which include application of ac line voltage to loads such as heavy filament tungsten lamps, heater circuits or the like, include components which compensate for the inability of these loads to respond to the rapid power fluctuation of the line source. Various voltage regulation techniques are known in the art; the most commonly being to electrically apply and remove the load power by selectively controlling the operation of a silicon- controlled-rectifier (SCR). The SCR operation, in turn, is usually controlled by using an optical feedback circuit. Systems of this type are disclosed in U. S. Patents 3,210,647, 3,952,242 and 3,670,202.
Each of these systems, however, encounters the problem of compensating for the effects of electromagnetic interference (EMI) caused by the rapid switching times associated with SCRs (typically 1 to 2µ5). The EMI effects are manifested in fast rise times in the load voltage which must be eliminated. One method of eliminating the EMI effect is to insert filters between the SCR and the load as shown, for example, in U. S. 3,670,202. This represents an additional, and costly, component to these systems and increases their complexity. The use of filters also creates an undesireable AC ground leakage current which must be limited for safety reasons.
The present invention is directed to a power regulation system which applies and removes power from a load "slowly", thereby reducing the EMI and eliminating the need for filters. A transistor, rather than an SCR, is then used to control the rate at which power is applied to the load. A circuit according to the invention is characterised by: rectifier means for
converting alternating current power to full wave rectified direct current power; means for supplying said rectified power to the load; means for generating a first output signal V representative of the radiant energy of said load; means for comparing said output V with a portion of said rectified dc power to produce a second output V ₀; a transistor switching circuit coupled between said comparing means and said load, said switching circuit being adapted to be turned on and off at a rate determined by the rate of change of the level of output V ; whereby said load is turned on and off consistent with the switching rate of said transistor switching circuit.
In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which:-

Figure 1 is a block diagram of an embodiment of the present invention.
Figure 2 is a detailed schematic diagram of a first embodiment of the Figure 1 system.
Figure 3 represents line voltage and load waveform levels plotted against time.
Figure 4 is a detailed schematic diagram of a second embodiment of the Figure 1 system.

Referring to Figure 1, a phase-controlled, regulated power supply circuit is shown to consist of a number of functional components 10, 20, 30, 40, 50, 60. A conventional ac power source 10 is connected to rectifier 20. Full wave rectified de signals are applied across lamp 30 and transistor switch 40. The operation of transistor switch 40 is controlled by a feedback loop consisting of photosensor 50 and comparator-amplifier 60. Photosensor 50 generates an output signal proportional to the intensity of light emanating from lamp 30. The signal is amplified and compared, in comparator-amplifier 60, with a fraction of the rectifier 20 output. The output of comparator-amplifier 60 is a voltage signal which changes at a rate which controls the operation of transistor switch 40, and, hence of lamp 30.
Figure 2 is a specific embodiment of the circuit shown in Figure 1. A ll5v 60 Hz line voltage from source 10 is applied to conventional dc bridge rectifier 20 which includes four diodes 20a, 20b, 20c and 20d, connected as shown. Rectifier 20 converts the applied ac power to full wave rectified dc power across output leads 22, 24. The output on lead 22 has a 120 cps ripple component which is filtered by a circuit comprising capacitor 26, zener diode 27 and resistor 28. This filtered de voltage is then used to provide power, along lead 25 to the amplifier circuits and a biasing signal to circuit 60, as will be seen.
The full wave, rectified dc output on lead 22 is applied to a 100W, 100V tungsten lamp 30 in series with transistor switch 40 consisting of Darlington transistor 42 and resistors 44, 45. The function of transistor 43 is to provide an overcurrent shutdown of the output to protect transistor pair 42 during warmup of the lamp. Photosensor circuit 50 which monitors the light output of lamp 30, includes a photodiode 52, and operational amplifier 54. Photodiode 52 generates an output current proportional to the irradiance from lamp 30 impinging thereon. Capacitor 56 filters out any ripple components. Amplifier 54 provides a current-to-voltage conversion of the photodiode signal and its output signal V_L is proportional to the input photodiode signal and the gain of the circuit (controlled by potentiometer 58).
Voltage signal V_L is applied to the positive input of operational amplifier 61 in amplifier-comparator circuit 60. The rectified, filtered output from lead 22 is applied to the negative input of amplifier 61 across a voltage divide comprising resistors 62, 63. Amplifier 61 output signal, V_c, is proportional to the input signal from amplifier 54 and the gain of the circuit as set by resistor 64. For the selected component values, amplifier 61 provides a non-inverted voltage output with a gain of approximately 10.
The voltage output, V , of amplifier 61, is applied to the negative input of operational amplifier 65 across resistor 66. This input is compared with a fraction of the rectified line voltage on lead 22, applied to amplifier 65 across a voltage divider consisting of resistors 67, 68. The output V_o of amplifier 65 is therefore proportional to the input signal from amplifier 61 and the gain as set by resistor 69. For the indicated component values, amplifier 65 provides a non-inverted voltage output V with a gain of approximately 11. This signal is applied to transistor switch circuit 40. As will be seen, the rate at which output V changes (along with the value of resistor 44 and the gain of transistor 42) affects the rate at which the lamp current changes.
In operation, Darlington transistor pair 42 will be biased into conduction whenever the value of V₀ goes positive. Output V₀ will go positive whenever the input to the positive terminal of amplifier 65 is sufficiently positive with respect to the input V_cat the negative terminal. At initial turn-on conditions, V will be a minimal value since current is not flowing through lamp 30 and photodiode 52 is not detecting any light output. Referring to Fig. 3, under these conditions, V₀ will go positive at some point A on the full wave waveform at some time value k (t) after full power application. Since V goes positive, transistor 42 is biased into conduction and power is applied to lamp 30. Transistor 42 will continue to conduct until the bias voltage to amplifier 65 drops below the preset value at A', at which point transistor 42 is turned off. Lamp 30 has therefore been energized for some period t_l and has produced a light component which is detected by photodiode 52. The amplified signal will be present as signal V_c at the negative input of amplifier 65. Amplifier 65 will therefore not go positive until it sees a greater line voltage input B at the positive terminal. Transistor 42 will then be turned on and conduct during a shorter time period t₂ terminated at point B', where the line voltage drops below the value of V . Again, the photosensor signal generates a signal proportional to the increased light output of lamp 30. It can then be appreciated that with the application of each successive line voltage waveform, up to the point where equilibrium conditions are achieved, V_c will continue to increase causing V to go positive at greater values of the applied line voltage. Waveform C illustrates a typical waveform for normal lamp operating conditions for the circuit shown in Figure 2, and with a current gain of 200 for transistor 42, lamp current will increase from 0 to 1 ampere when the line voltage has a net change of approximately 10 volts. The time required for the line voltage to change by 10 volts depends on the shape of the line voltage waveform but at the steepest part, the time required for a 10 volt change is 165µ sec.
Specific values for the components shown in Figure 2 are listed below:

FIGURE 2 COMPONENTS

Referring now to Figure 4, the circuit of Figure 2 has been modified by introducing an additional amplifier in the trigger circuit and using the Darlington transistor as the output. The circuit operates in the same manner as described above producing voltage V at the output of comparator-amplifier 65. This drive voltage is divided down by resistors 70 and 71 to provide a low-level, control voltage to amplifier 72. Amplifier 72 drives Darlington transistor pair 73 via resistor 74 so that the current from lamp 30 causes a voltage drop across resistor 75 equal to the control voltage. The voltage gain of amplifier 72, as set by resistors 76, 77, equals 150, so that sufficient base-drive voltage will always be present to operate transistor pair 73, regardless of its current gain. Transistor 78 serves the same function as transistor 43 in Figure 2. It is therefore seen that, since the output signal V is an approximate representative of the desired output current, the introduction of high gain closed loop transconductance (voltage to current converter) amplifier 72 removes the undesirable effects of current gain changes in the Darlington transistor.
Specific values for the transistor switch of Figure 4 are listed below:

FIGURE 4 (Switch 40) Components

From the above, it is seen that the lamp has been phased into a normal operating mode by controlling lamp turn-on and turn-off via a transistor switching circuit thereby reducing the electromagnetic affects associated with the more rapid SCR turn-on techniques.
While the above description has described a circuit wherein the load is a tungsten lamp, the principles of the present invention are applicable to the application of power to other types of loads such as radiant heaters, for example, of the type used to fuse xerographic images on a recording sheet. In this case, a sensing element which detects radiant heat emanating from the fuser could be used in place of the photodiode.

Claims

1. A phase-controlled regulated power supply circuit for controlling application of ac power to a radiant energy producing load (30) characterised by:

rectifier means (20) for converting alternating current power to full wave rectified direct current power,

means (22) for supplying said rectified power to said load (30),

means (50, 60) for generating a first output signal V representative of the radiant energy emanated by said load,

means (65) for comparing said output V_c with a portion of said rectified dc power to produce a second output V ,

a transistor switching circuit (40) coupled between said comparing means (60) and said load (30), said switching circuit being adapted to be turned on and off at a rate determined by the rate of change of the level of output V ;

whereby said load (30) is turned on and off consistent with the switching rate of said transistor switching circuit.

2. A power supply circuit according to claim 1, wherein said first output generating means (50, 60) includes a radiant energy sensing device (51) which generates a current proportional to the radiant energy impinging thereon,

a first amplifier means (54) for converting said sensing device current into a voltage ouput V_L, and

a second amplifier means (61) for amplifying said output V_L to generate said output signal V .

3. A power supply circuit according to claim 2, wherein said load (30) is a tungsten lamp and said sensing device (51) is a photodiode.

4. A power supply circuit according to claim 1, 2 or 3, wherein said transistor switching circuit (40) includes a Darlington transistor (42, 73).

5. A power supply circuit according to claim 4 wherein said switching circuit (40) further includes a high gain closed loop transconductance amplifier (72) connected between said comparator means (65) and said Darlington transistor (73).